Vanadium oxide cathode active material and method of making same

FIELD OF THE INVENTION

This invention relates to electrochemical batteries and more particularly to improved positive electrode active material mainly composed of vanadium pentoxide (V₂ O₅).

BACKGROUND OF THE INVENTION

The basic components of an Li/V_x O_y cell include a lithium anode, a separator, and a vanadium oxide cathode. The cathode is usually composed of an oxide of vanadium, such as vanadium pentoxide (V₂ O₅), and other components such as graphite and an electrolyte/binder.

During cell operation, incorporation of lithium in V₂ O₅ occurs. It is known that the Li/V₂ O₅ system heretofore in use has a multi-plateau discharge characteristic. Li/V₂ O₅ cells are known to have as many as 3 voltage steps above 2.0 volts. Gross plateaus appear at about 3.2 V, 2.4 V, and 2.1 to 2.2 V.

The relatively abrupt drop from about 3.0 volts to less than about 2.4 volts presents a major inconvenience for many electronic applications. Furthermore, it is known that the rechargeability of the Li/V₂ O₅ system is very sensitive to the lower voltage limit. If the battery is discharged to about 2.2 volts, the battery is practically impossible to recharge.

In U.S. Pat. No. 4,675,260, Sakurai et al describes binary and ternary cathode active materials of V₂ O₅ with at least one of P₂ O₅, TeO₂, GeO₂, Sb₂ O₃, Bi₂ O₃ and B₂ O₃ and/or at least one metal oxide selected from the group consisting of MoO₃ and WO₃. Precursors of such binary and ternary products are heated to a melt and rapid quenched. The discharge profiles of such binary and ternary products contain significant plateaus which indicates that such products still possess some order and are not fully amorphous.

What is needed is a new cathode active material based on V₂ O₅, in a form which maintains its capacity and has good charge/discharge characteristics. What is also needed is a method to form such V₂ O₅ -based materials.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a new cathode (positive electrode) V₂ O₅ -based active material for a lithium battery. Another object is to provide a method of making the new cathode material and the battery containing such cathode material. It is also an object to provide a lithium battery having a cathode active material containing an amorphous oxide of V₂ O₅, formed in situ. Another object is to provide a lithium battery having good charge and discharge capacity.

The present invention provides a lithium battery comprising a cathode active material having as its major component an amorphous V₂ O₅ vanadium pentoxide, which is lithiated amorphous vanadium pentoxide (Li_x V₂ O₅). Preferably, the active material consists essentially of lithiated amorphous vanadium pentoxide (Li_x V₂ O₅). The amorphous V₂ O₅ -based material is prepared by electrochemically reacting crystalline vanadium pentoxide (V₂ O₅) with lithium in a cell to a voltage sufficient to achieve the inclusion of more than two moles equivalent of lithium for each mole equivalent of the vanadium pentoxide. A suitable voltage is below about 2.0 V, desirably, a voltage of about 1.8 V or less and, preferably, a voltage of about 1.5 V or less.

Suitable voltage levels are in a range of about 1.5 V to about 2.0 V and, preferably, in a range of about 1.5 V to about 1.8 V. Preferably, the electrochemical reaction is conducted in a cell to a voltage in the range of about 1.5 V to about 1.8 V in a V₂ O₅ --Li cell at about room temperature and at a relatively constant current density. A suitable current density is less than 1 milliamp per cm² and, desirably, in a range of about 0.05 milliamps per cm² to about 0.5 milliamps per cm².

Preferably, the amorphous V₂ O₅ -based active material is prepared in, and used in, cells with an anode active material made of lithium or an alloy or a compound which includes lithium. The cells also include an electrolyte which is chemically stable with respect to the cathode active material (V₂ O₅) and lithium and which allows lithium ions to move therethrough to react electro-chemically with the cathode active material (V₂ O₅) of the invention.

A preferred lithium cell comprises the positive electrode of the invention, a negative electrode which is metallic lithium, and an electrolyte which is a solid electrolyte or electrolyte separator in the form of a polymeric network containing an electrolyte solution comprising a metal salt of lithium.

Advantageously, the method of the invention forms the amorphous vanadium pentoxide (V₂ O₅) in situ. Thus, the components of the cell are assembled once, the transformation of V₂ O₅ to an amorphous state is accomplished by the process of the invention, and then the cell is ready for use. For such in situ formation, the anode must contain both an amount of lithium needed in the battery (cell) during normal use and an additional amount corresponding to lithium needed for transformation of crystalline V₂ O₅ to amorphous V₂ O₅ and/or lithiated amorphous V₂ O₅ (Li_x V₂ O₅).

These and other objects, features and advantages will become apparent from the following description of the preferred embodiments, appended claims and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a cross-section of a lithium battery or cell embodying the invention.

FIG. 2 is a voltage curve of the first discharge of a conventional crystalline V₂ O₅ and lithium battery .

FIG. 3 is a voltage curve of the second discharge cycle of amorphous V₂ O₅ cathode material of the invention, coupled with a metallic lithium anode.

FIG. 4 is a curve showing the rechargeability of the amorphous V₂ O₅ positive electrode active material of FIG. 3, demonstrated for a number of cycles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, an electrochemical cell or battery 10 has a negative electrode side 12, a positive electrode side 14, and a separator 16 therebetween. In accordance with common usage, a battery may consist of one cell or multiple cells. The negative electrode is the anode during discharge, and the positive electrode is the cathode during discharge. The negative electrode side includes a current collector 18, typically of nickel, iron, stainless steel, and/or copper foil, and a body of negative electrode active material 20 made of lithium or an alloy of lithium or a compound which includes lithium. The negative electrode active material 20 is sometimes simply referred to as the negative electrode. The positive electrode side includes a current collector 22, typically of aluminum, nickel, iron, stainless steel, and/or copper foil, and such foils having a protective conducting coating or foil, and a body of positive electrode active material 24 which has as its main component V₂ O₅. The positive electrode active material 24 is sometimes simply referred to as the positive electrode. The separator 16 is typically a solid (polymer) electrolyte, electrolyte separator. A suitable electrolyte separator (metal salt/polymeric material separator) is described in U.S. Pat. No. 4,830,939, and is a solid organic polymer matrix containing an ionically conducting liquid with an alkali metal salt and the liquid is an aprotic polar solvent. Typical separator materials also include glass, cellulose and polymers such as PVC which separate electrodes and permit migration of electrolyte components. A lithium battery must be non-aqueous, thus electrolyte components include a lithium metal salt and aprotic solvent.

As mentioned, the rechargeability of a lithium battery may be limited by the cyclability of the cathode (positive electrode) active material. In the case of batteries heretofore in use employing crystalline V₂ O₅ during cell operation, incorporation of lithium in V₂ O₅ occurs. More particularly, as shown in FIG. 2, it is known that this system has a multi-plateau discharge characteristic. Li/V₂ O₅ cells are known to have as many as 3 voltage steps above 2.0 volts. Gross plateaus appear at about 3.2 V, 2.4 V, and 2.1 to 2.2 V (FIG. 2). The relatively abrupt drop from about 3.0 volts to less than about 2.4 volts limits its applicability. Furthermore, it is known that the rechargeability of the Li/V₂ O₅ system is very sensitive to the lower voltage limit.

Surprisingly, it has been found that when the discharge of V₂ O₅ -based cathodes is continued down to a certain voltage range, a structural change of the V₂ O₅ takes place. This transformation results in an amorphous V₂ O₅ which is lithiated and which, when used as the active material of a positive electrode, results in a battery having consistently high capacity and good charge/discharge characteristics. Preferably, the active material consists essentially of the amorphous vanadium pentoxide (V₂ O₅) and/or lithiated amorphous vanadium pentoxide (Li_x V₂ O₅). The amorphous V₂ O₅ is prepared by electrochemically reacting crystalline vanadium pentoxide (V₂ O₅) with lithium in a cell to a voltage below about 2.0 V, desirably, to a voltage of about 1.8 V or less and, preferably, to a voltage of about 1.5 V or less. Suitable voltage levels are in a range of about 1.5 V to about 2.0 V and, preferably, in a range of about 1.5 V to about 1.8 V. Preferably, the electrochemical reaction is conducted in a cell to a voltage in a range of about 1.5 V to about 1.8 V in a V₂ O₅ --Li cell at about room temperature and at a relatively constant current density, less than 1 milliamp per cm² of the electrode. Desirably, the current density is in a range of about 0.05 milliamps per cm² to about 0.5 milliamps per cm². The area used to derive current density is the surface area of one electrode which is the same as the active area of a cell or battery having an anode and a cathode. A broad range of temperatures around a typical 20° C. room temperature are suitable, such as 5° C. to 35° C.

Several methods may be used to form the crystalline V₂ O₅ starting material. In one method, the crystalline V₂ O₅ (vanadium pentoxide) is prepared from ammonium metavanadate (NH₄ VO₃). The ammonium metavanadate is heated to a temperature of about 400° C. to about 450° C. to decompose it to crystalline V₂ O₅ in the presence of oxygen. The crystalline V₂ O₅ was used to prepare cells with lithium-based anodes. Several cells were prepared by mixing V₂ O₅, carbon (typical Shawinigan black) and electrolyte/binder. A typical composition which was used for most of the experiments is as given in Table 1.

<pre xml:space="preserve" listing-type="tabular"> <!--Greenbook tabular data--> TABLE 1______________________________________Typical Cathode Composition Percent by Weight______________________________________V₂ O₅ 45.2%Carbon 10.3%Propylene Carbonate (PC) 33.3%PolyEthylene Oxide (PEO) 1.0%PolyEthyleneGlycolDiAcrylate (PEGDA) 8.8%TriMethylPolyEthylene Oxide TriAcrylate 1.5%(TMPEOTA)______________________________________</pre>

The cathode was coated onto nickel foil followed by electron beam curing (cross-linking/polymerization) of the acrylate component. Then the electrolyte was coated on top of the cathode and cured with ultraviolet light. The lithium electrode was applied on top of the electrolyte separator and the battery was finally placed in a flexible pouch which was heat sealed under vacuum.

The crystalline V₂ O₅ was converted to amorphous V₂ O₅ by discharging each battery (cell) to about 1.5 V at room temperature. This yielded good results. However, the exact voltage level for the conversion is thought to be in a range of about 1.5 to about 2 V. The current density was about 0.25 mA/cm² of the electrode in most cases, but at higher densities, the conversion was also observed although, sometimes over a couple of cycles.

The transformation from crystalline to amorphous V₂ O₅ created a new, x-ray amorphous compound, which readily released at least a portion of inserted lithium. The transformation of the V₂ O₅ created a rechargeable cathode (positive electrode) material with a high capacity. Furthermore, the discharge curve of the new material was smooth, featureless, and sloping as shown in FIG. 3. FIG. 3 is the second cycle of the battery of the invention having the new amorphous V₂ O₅ -based cathode of the invention. Voltage curves for the third and following cycles were obtained and found to be virtually identical. The rechargeability of the new material was good as demonstrated for multiple recharge cycles as shown in FIG. 4. Compared to crystalline V₂ O₅ the rechargeable capacity of the amorphous V₂ O₅ of the invention is higher and over-discharge is not a problem. In contrast, over-discharge of crystalline V₂ O₅ destroys the battery.

It is possible to use lithium alloys in the cell, rather than metallic lithium, provided the voltage is adjusted accordingly. For example, in the case of LiAl, the voltage range would be 0.38 V lower than with pure lithium. Other alloys are Li_x Si, Li_x Bi, Li_x Sn, Li_x Pb and Woods (bismuth-cadmium) alloys. Also any other anode alternative to lithium, including lithium-containing compounds such as Li_x C₆, lithium containing organic redox polymers, and lithium insertion and intercalation materials may be used, provided the anode contains the lithium of the battery system, and the voltage requirement is observed, as described below. In such cases, the crystalline V₂ O₅ is converted to amorphous V₂ O₅ by discharging the batteries to less than about 1.5 V, and sometimes approaching 1.0 V. In any event, the transformation from crystalline to amorphous form of the V₂ O₅ is thought to be achieved when a particular molar equivalent of Li is electrochemically reacted with crystalline V₂ O₅.

Although not wishing to be held to any particular theory, it is thought that the following applies to the conversion of crystalline V₂ O₅ to amorphous V₂ O₅ . This conversion is not so much determined by the voltage of the cell as by the lithium content of the V₂ O₅. The amount of lithium which is needed to initiate the transformation is not well understood. In order to achieve good results, it is, therefore, easier to refer to and to monitor cell voltage using a relatively low current density. By this means, sufficient lithium is inserted in the V₂ O₅ to achieve the desired results. The amount of lithium needed is thought to be more than 2 moles equivalent of Li+ per each mole equivalent of V₂ O₅, but the transition from the crystalline to the amorphous state is thought to take place before 3 moles equivalent of Li+ per each mole equivalent of V₂ O₅ is reached. There seems to be a very broad range. In the case of a current density less than about 1 mA/cm² of the electrode, it is possible to convert practically all crystalline V₂ O₅ to the amorphous form by discharging very close to 1.5 V, when a pure lithium anode is used. With LiAl alloy, the voltage limit is about 1.1 V at the same current density. However, it is also possible to achieve the transition at higher current densities, but the conversion may take place over several discharge/charge cycles. In the case of an electrolyte with a relatively high conductivity, the same may be achieved in one cycle at the relatively high end of the current density range. While it is thought that lithium ions (Li+) are essentially inserted (intercalated) into the vanadium oxide, the mechanism is not well understood. Therefore, it is possible that cleavage of bonds of the host V₂ O₅ may occur along with disturbance (perturbation) of bond arrangement of the host. As a result, the lithiated amorphous vanadium oxide of the invention may contain some lithium irreversibly bonded to the oxide as well as some lithium occupying sites within the oxide host without such bonding (essentially reversible insertion).

It is thought that the maximum amount of lithium which the lithiated V₂ O₅ can intercalate is approximately equivalent to the general formula Li₃ V₂ O₅. It may be 5-10% higher, but it is unlikely; and it is even less likely to be equivalent to Li₄ V₂ O₅. Assuming a maximum of three moles equivalent of Li+ ions for each mole equivalent of V₂ O₅ (i.e. 3 Li+ ions per each V₂ O₅ atom; 3 Li+/V₂ O₅), and referring to FIG. 4, the fully charged amorphous Li_x V₂ O₅ contains lithium corresponding to x=0.75. The amount of x may be smaller under other charging conditions, but is the approximate amount which was left when a battery was cycled between 1.5 and 3.5 V at a current density of 0.25 mA/cm². Both charge and discharge occurred at room temperature.

While this invention has been described in terms of certain embodiments thereof, it is not intended that it be limited to the above description, but rather only to the extent set forth in the following claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined in the appended claims.